It's a mythological reference: "In ancient Greek religion and mythology, Pluto (Greek: Πλούτων, Ploutōn) was the ruler of the Greek underworld." (
https://en.wikipedia.org/wiki/Pluto_%28mythology%29). As someone else remarked in a related thread elsewhere, we already have his dog (
https://en.wikipedia.org/wiki/Kerberos_%28mythology%29) as the name of a security protocol.